The use of speech recognition has become widespread in recent decades. For example, speech recognition often is used in telephone call centers to process menu selections uttered by callers. Speech recognition also is frequently used by devices to receive user commands. For instance, mobile telephones typically are configured to receive voice inputs to initiate a telephone call, as well as access other telephone functions. Vehicle navigation systems also may include speech recognition to process user requests for directions. Further, modern computer applications sometimes include voice recognition as an option for converting spoken utterances into text that is incorporated into word processing documents and other electronic files. Still, there are a myriad of other devices and systems that implement speech recognition, and the number of such devices/systems continues to grow.
Speech recognition systems sometimes have difficulty interpreting certain types of words, such as names that are relatively unique. Navigation systems, in particular, oftentimes have difficulty in recognizing utterances that identify points of interest with words other than known street names. In such circumstances, a user may be prompted to utter letters to spell one or more words to identify a point of interest. Such utterances are considered an unconstrained sequence since potential permutations of letters are not bounded to a few likely combinations. Moreover, in comparison to words, letters are very difficult to recognize using speech recognition. Accordingly, unconstrained letter recognition is very difficult to implement with a high level of accuracy. In consequence, words interpreted in this manner frequently are misidentified, resulting in much user frustration.